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Dry Matter, Nitrogen Distribution, and Dinitrogen Fixation in Contrasting Alfalfa Symbioses¹

Improving dinitrogen (N₂) fixation of alfalfa (Medicago sativa L.) requires combining effective rhizobial strains and host genotypes capable of enhanced N₂ fixation. Glasshouse experiments on pot-grown alfalfa determined how changes in the rhizobial and host genomes associated with N₂ fixation affect dry matter and N yields and distribution at vegetative, flower bud, and 30% flowering developmental stages. We compared dry matter and N yield and distribution in a host- and a rhizobial-conditioned ineffective (Fix⁺) and in a related effective (Fix⁺) symbiotic combination, and investigated the effect of combined N on N₂ fixation and on the distribution of symbolically fixed N₂ in two unrelated Fix⁺ alfalfa germplasms. Dry matter and N yield and response to added N (0, 40, 80 kg/ha) of the host-conditioned and the rhizobial-conditioned Fix^– combinations were similar. Without combined N, both Fix^– combinations distributed relatively less dry matter and N to aerial organs than did the Fix⁺ combination. Neither the type of Fix^– combination nor traits in the Fix⁺ germplasms supportive of increased N₂ fixation altered the relative distribution of dry matter among plant organs; however, N concentration in individual organs varied with supplemental N in the Fix^– but not the Fix⁺ combinations. Dry matter and N yields of the two unrelated Fix⁺ combinations differed only without combined N. Supplemental N was more inhibitory to N₂ fixation by Fix⁺ germplasm selected for enhanced nitrogenase activity compared with unselected Fix⁺ germplasm. Either source of ineffective nodulation in Fix^– combinations would be appropriate for developing non-fixing controls for physiological investigations of N₂ fixation and for use in programs to evaluate and enhance N₂ fixation in alfalfa.

Key Words: Medicago sativa L. • Nitrogen nutrition • Host-rhizobial interaction • Ineffective nodules • Nitrogen-15 • Source-sink relations

² Office of USDA. Received 26 Dec. 1987. 2 Assistant professor, Dep. of Biology, St. Olaf College, Northfield, MN 55057, and plant physiologists, USDA-ARS, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108.

Received for publication December 26, 1987.